This invention relates to apparatus for monitoring the performance of an electric motor.
It is known to monitor the performance of an electric motor by detecting stray magnetic flux from the motor and providing corresponding electrical signals, performing a frequency analysis of the flux signals and providing diagnostic information in dependence on variations in the flux signals. For example, for a motor powered by a 50 Hz supply, the amplitude of flux signal components at 25, 50, 100 and 150 Hz can be monitored and an alarm provided in the event of a large and sudden change. A possible frequency analysis system involves sampling the magnetic flux to determine its amplitude at a number of points within a mains cycle and processing the samples with a digital processor to determine the amplitudes of the individual frequency components of interest. In such a system the timing of the sampling points has a significant effect on the accuracy of the processing and it is important to sample the flux signals at times which are accurately related to the fundamental cycle, i.e. to the mains supply cycle. The sampling times are determined by the digital processor and it is therefore important to synchronise accurately the operation of the digital processor to the mains supply, even during possible variations in the frequency of the mains supply.
It is known to use a phase-locked loop to synchronise digital circuitry to an external signal but, particularly when there is a large ratio between the operating frequency of the digital circuitry and that to which the circuitry is to be synchronised, the cost of the synchronisation circuit is high. It is important to use low-cost circuitry in a motor monitor because it is desirable that the cost of the monitor should be small compared to that of the motor itself.